Technical Field
The present disclosure relates to electrosurgical apparatuses, systems and methods. More particularly, the present disclosure is directed to electrosurgical systems configured to measure impedance using resonance phasing during electrosurgical procedures.
Background of Related Art
Energy-based tissue treatment is well known in the art. Various types of energy (e.g., electrical, ultrasonic, microwave, cryogenic, heat, laser, etc.) are applied to tissue to achieve a desired result. Electrosurgery involves application of high radio frequency electrical current to a surgical site to cut, ablate, coagulate or seal tissue. In monopolar electrosurgery, the active electrode is typically part of the surgical instrument held by the surgeon and applied to the tissue to be treated. A patient return electrode is placed remotely from the active electrode to carry the current back to the generator and safely disperse current applied by the active electrode.
The return electrodes usually have a large patient contact surface area to minimize heating at that site. Heating is caused by high current densities which directly depend on the surface area. A larger surface contact area results in lower localized heat intensity. Return electrodes are typically sized based on assumptions of the maximum current utilized during a particular surgical procedure and the duty cycle (i.e., the percentage of time the generator is on).
The first types of return electrodes were in the form of large metal plates covered with conductive jelly. Later, adhesive electrodes were developed with a single metal foil covered with conductive jelly or conductive adhesive. However, one problem with these adhesive electrodes was that if a portion peeled from the patient, the contact area of the electrode with the patient decreased, thereby increasing the current density at the adhered portion and, in turn, increasing the heating at the tissue site.
To address this problem various return electrodes and hardware circuits, generically called Return Electrode Contact Quality Monitors (RECQMs), were developed. Such systems relied on measuring the external impedance at the return electrode to calculate a variety of tissue and/or electrode properties. These systems detected peeling by identifying changes in amplitude of the impedance of the return electrodes.
An electrosurgical generator typically uses an interrogation signal to continuously measure the external impedance, without activating the generator. This is depicted with respect to generator 100 shown in FIG. 1. The interrogation signal (Sense) monitors the impedance external to the generator 100 while the generator's main power (Gen) is disconnected from the output. The interrogation signal is utilized in various monitoring circuits, such as RECQMs, Return Electrode Monitoring (REM) and Auto Bipolar sensor (ABP), that continuously monitor external impedance while the generator is inactive.
A more detailed example of an impedance sensing circuit is the REM circuit shown in FIG. 2 which is used in various commercially available generators and designated by reference numeral 200. The REM circuit 200 is a resonance circuit consisting of capacitors and a transformer within a generator 205. The circuit 200 resonates around 80 KHz; the same frequency as the REM clock (REM CLK) generated by controller board 210 for driving impedance detection circuitry 220. The controller board 210 also drives amplifier 230. The REM circuit 200 drives the interrogation signal into a split pad (REM PAD) 240 that is attached to the patient. The impedance detection circuitry 220 filters and rectifies the interrogation signal. The interrogation signal represents the magnitude of impedance (|Z|) that is across the REM pad 240.
The REM pad 240, in conceptual terms, is a parallel plate capacitor, which means the impedance of the pad 240 has a resistive and reactive part to the impedance. The reactance is the capacitive coupling and the resistance is the dielectric losses between the pads. FIG. 3 graphically represents the relation between the magnitude |Z|, the real R, and reactance X part of the impedance. Just measuring the magnitude will not give enough information to determine how much coupling versus the losses there is between the patient and the pad 240.
An example of where this information would be useful: if a REM pad is folded upon itself and only a small section is touching the patient, the existing REM circuit could give a false green indication. By knowing the reactance and resistance in this case, the generator would see a very high capacitance versus the resistance and be able to determine a fault condition.
The REM circuit 200 has a single frequency that is used to monitor the magnitude of the resonance circuit (REM CLK in FIG. 2). Any reactance placed in line with the REM circuit will shift the resonance frequency causing the magnitude at the monitoring frequency to reduce. Since the REM circuit 200 is only monitoring at one frequency, this shift will move the actual magnitude of the signal and the REM circuit 200 is left measuring the tail end of the shifted signal. This is shown in FIG. 4, where Vr_oc is the open circuit voltage of the REM circuit 200 when no pad attached; Yr_load is the actual voltage with a pad attached; and Yr_REM is the voltage the REM circuit 200 measures.
If a user adds the correct amount of capacitance in series with the single pad return electrode, the resonance frequency of the REM circuit 200 will shift, such that the REM circuit 200 will measure a valid impedance and override the REM circuit 200, thus defeating any safety mitigation.
Additionally, any drift in the resonance of the REM circuit can be an issue. As components heat and age over time there is a possibility of the resonance shifting from its original frequency. To date, it is believed there is no reliable way of tracking this change and/or compensating for any change that may occur.